Magnetic field permeable barrier for magnetic position measurement system

ABSTRACT

A magnetic field position and orientation measurement system contains, confines and re-directs the magnetic field from one or more transmitters such that the fields are attenuated in areas outside of the operating volume in areas where metallic objects are commonly found. A thin barrier made of a highly permeable material such as ferrite or mumetal is placed on top of a conductive plate. The thickness of the permeable layer is from 0.01 inches to 0.25 inches while the conductive plate, preferably made of an aluminum alloy, may preferably be from {fraction (3/16)} of an inch to ¼ inch in thickness. On top of the permeable barrier, a rhombic three axis transmitter is placed. In the preferred embodiment, the transmitter consists of a PC board carrying the transmitter. PC boards having thicknesses varying from 0.03125-0.125 inches may be employed. Thus, the entire “stack” including the transmitter, the permeable barrier and the conductive plate may only be from ½ inch to ⅝ of an inch in thickness. The permeable barrier may have a flat, planar configuration. Alternatively, it may be made to resemble, in cross-section, a cake pan having a flat central region with uplifted peripheral edges. Alternatively, the permeable barrier may have a generally flat configuration with peripheral edges that taper outwardly from the top surface thereof to the bottom surface thereof with the taper making an angle with the bottom surface in the range of, preferably, 30° to 85°.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic position measurement systemwith field containment means. The concept of using transmitting andreceiving components with electromagnetic coupling is well known withrespect to biomechanics and medical diagnostics, wherein a sensorassembly is mounted on a point of interest and the position of the pointis determined relative to a fixed transmitter. This information is thenused by computing systems to precisely show the relative motions of thepoints in question, which, in the medical sense, allows instruments tobe precisely located in a human body with respect to the body and eachother. This allows new, advanced methods of surgery and diagnostics tobe performed.

When conductive materials are present, which is often the case on,below, or near an operating table, they generate eddy current fields,which distort the received magnetic field waveform, which distorts theoutput of the system unless the system utilizes some distortion reducingor compensating technique. When permeable materials are present, theybend and otherwise distort the magnetic field, with effects similar toconductive materials. In a surgical theater, both conductive andpermeable materials are present in substantial quantities. They are amajor component of many operating tables, surrounding equipment such ascarts and equipment, and are present in the movable spot lamps used toilluminate the surgical field. Many operating tables have many degreesof positional and angular freedom to allow optimal placement of thesurgical field relative to the surgeon, and are designed to be extremelystable and sturdy while supporting a heavy human body. As a result ofthese requirements, the tables contain numerous mechanisms allowingfore, aft, up, down, sideways, roll, and tilt motions. These mechanismsare physically robust and typically fabricated from steel, so that theyhave substantial field distortion characteristics. Shapes may includescrews, rack and pinion gears, or scissors type actuators. The tablesurface may be one piece, or may be divided into several sections, witheach section capable of motion relative to the other sections, to allowa body to be flexed such that various stresses and relative anatomicalpositions are optimal for a particular surgical or diagnostic procedure.The installed bases of operating tables are extremely diverse in design,and as the tables are often in service for many decades, there are manyvendors, with each vendor carrying a number of different operating tabledesigns. This poses a significant problem for magnetic position trackingsystems which are used in a critical surgical environment. The operatingvolume for the tracker is typically within the body which lies on top ofthe table. This means that the tracking system is operating in closeproximity to the metallic structures on, under, and around the table.The magnetic fields are distorted by these structures, which may resultin large errors in the reported magnetic sensor position. The largediversity in table designs makes it impossible to predict the severityof distortion experienced on a given table. This is an unacceptablecondition for a surgical environment. Attempts to compensate for thesedegrading effects have been made with varying degrees of effectiveness.

One method already employed is to map the entire operating volume eachtime the system is used. This is very time consuming and expensive, aspotentially thousands of points must be taken in a precise manner if thedistortion is severe and the operating volume large. It is alsounreliable since during a surgical or diagnostic procedure, the tablegeometry is often changed which changes the relation of the tablemetallic structures relative to the tracking system, thereby requiring anew map if errors cannot be tolerated. Instruments and diagnosticequipment are also introduced and removed from the vicinity of thetracking system, thus rendering a map ineffective. Also, for severedistortion, a map may become totally ineffective, as the system may, attwo different physical sensor spatial points, determine the sensor to beat the same position. In this case, the output data is of minimal use.

Another known method commonly described in prior art is to use AC fieldsover a conductive ground plane. The ground plane attenuates the magneticfield below the plane to nearly zero, which has the benefit of makingthe system insensitive to metallic objects below the plane. In the caseof a dipole transmitter, the “method of images” is used to compute thetheoretical magnetic field vectors over the plane, which are then usedto provide sensor position. This method has drawbacks. One is that nearthe ground plane, the magnetic field intensity is nearly zero, and thevector crossing angles are degraded, which seriously reduces systemperformance with respect to accuracy and noise. The net result is thatthe sensor must be kept a few inches above the plane. Also, the dipolemust be located some distance from the ground plane in order to reducesignal losses and degraded vector crossing angles within the operatingvolume. For a 1 cubic foot volume, the bottom of the transmitter must beabout 2 inches above the plane for acceptable performance. To computethe height at which a patient must be elevated if lying on thetransmitter, the thickness of the transmitter must be added to this 2inch figure. Transmitter size is determined by required signal levelwithin the operating volume. Sensor coil size for minimally invasivesurgical applications is about 1 mm×5 mm in cross-section, which is verysmall. The requirement for precise, low noise operation at the extremeedges of the volume requires that a relatively large magnetic fieldmagnitude be present in order to induce sufficient signal in the smallcoils. Transmitter size is largely dictated by how much field it mustoutput. Since the transmitter is typically a cube, to obtain sufficientsignal within a 1 cubic foot volume with a small receiver coil, thepractical transmitter dimensions are on the order of 2 inches per side.We can now see that the effective transmitter assembly in this prior artteaching, including the ground plane, is 4 inches thick. In a surgicalenvironment, the patient must be elevated to levels which a surgeon mayfind uncomfortable. In addition, extra padding may become necessary ifthe patient must lie flat on the table. Both the transmitter and thepadding must be secured to the table. In short, the configuration iscumbersome and may not allow the patient to be positioned in an optimalmanner.

Placing the transmitter above the operating volume is not desirable asit will potentially interfere with the surgical field. Also, as thetransmitter is placed further from the ground plane, and if thedimensions of the ground plane are fixed to be a square of about 18inches on a side, the ground plane becomes ineffective at reducing theeffects of metallic objects near the operating volume. The metalhousings of the surgical lighting equipment will have a greaterdistorting effect in the upper portions of the operating volume, as theyare closer to both the transmitter and receiver. Equipment used duringthe procedure, including the operating table, will cause potentiallylife threatening distortion, which is an unacceptable condition.

Position determination depends on relative vector magnitudes from the x,y, and z coils. Distortion effects may again be removed by using aprocess such as mapping. As the magnitudes of the transmitted magneticvectors from the x, y, and z coils become more similar, a given fixedamount of error in their determination will result in an increased errorin position output. Again, considering the limiting case, if themagnitudes become equal then position determination is not possible.This combined effect of reduced angle of transmitted vector intersectionand reduced difference in transmitted vector magnitudes is known tothose skilled in the art as vector dilution. Use of a conductive groundplane under the transmitter will cause vector dilution. The severity ofthe vector dilution is increased as the transmitter becomes closer tothe ground plane, and is also increased as the receiver becomes furtherfrom the transmitter. Vector dilution generally imposes a practicallimit on how close the transmitter of a magnetic tracking system may beplaced to a conductive ground plane. For a 1 cubic foot motion box,vector dilution approaches unacceptable levels if the transmitter isplaced closer than 2 inches from an infinite extent conductive groundplane. Vector dilution is also present in non-dipole transmitterconfigurations, and the effects of its presence are similar.

The Following Prior Art is Known to Applicant

U.S. Pat. No. 4,849,692 to Blood discloses a method of eliminating eddycurrent distortion effects, which are generated by conductive objects,such as the stainless steel table surface, and in other objects havinglarge surface areas. The distortion effects of permeable metals are notaddressed by this system. This means that steel structures in, around,and under the operating region of the system will distort the receivedmagnetic fields and degrade system performance. In addition, large,thick sheets of conductive metals such as Aluminum have eddy currentdecay times which can exceed 200 milliseconds. If the system uses 3 timedivision multiplexed transmit axes plus one period where all axes areoff in order to compensate for the earth's field, as described in thepreferred embodiment, this means that the update rate is ¼*(200 mS)=1.25Hz. This is unacceptably slow for many applications.

U.S. Pat. No. 5,767,669 to Hansen, et al. describes methods for eddycurrent field compensation without the need to compensate for theEarth's field effects. This system has no provision for reducing theeffect of nearby permeable metals, nor does it address the drawback ofrequiring a slow update rate while operating near large, thick sheets ofhighly conductive metals.

U.S. Pat. No. 5,600,330 to Blood discloses a non-dipole looptransmitter-based magnetic tracking system. This system shows reducedsensitivity to small metallic objects in the operating volume, as thefield from the smaller object will fall off as 1/r{circumflex over ( )}3with r being the received distance from that object, while the fieldfrom the larger transmitting loops will fall off as 1/r{circumflex over( )}2, which yields a reduced effect from the small metallic object.Large sheets of metal, however, can have an effective loop area largerthan the magnetic transmitter loops, which diminishes this advantage infield fall off rate, which has the general effect of making the systemquite sensitive to large metallic objects. Also, metallic objectsparallel to and near the transmitter loops produce very large eddycurrent magnitudes which reduces the signal level within the operatingvolume. In order to reduce the effects of metallic objects near thetransmitter in this system, the transmit coils must be placed somedistance away from the ground plane in order to reduce signal loss,which occurs when a loop of wire gets close to a conducting ground planeparallel with the plane of the loop. In the case of the planartransmitter configuration in this system, a planar ground plane may beplaced some distance below the transmit coils. For zero distance, themagnetic field reduction within the operating volume is nearly total, soone must find a compromise between effective transmitter thickness,defined as the total thickness of the transmit coils, ground plane, andspacing between them, and signal loss. Also, due to the fact that theground plane eddy current loop area is large with respect to a singletransmit coil area, there is an additional degrading effect as thesensor gets further from the transmitter. The ground plane currentdistribution is similar no matter which transmit coil is operating. Thismeans that the ground plane eddy current field vectors will be similaralso. Since the field at any point within the operating volume is thevector sum of transmit coil field minus ground plane eddy field, and theground plane field effective radius is larger than the transmit coilradius, we can see that the further we get from the plane of thetransmitter, the more the field is determined by the ground planecurrents. The net effect is that the vectors from the 3 transmit coilsare less distinct, which makes the system more sensitive to noise andmetallic distortion, as the system uses differences in the vectormagnitudes and directions to determine position. As these differencesbecome small, a small change on one of the vectors can result in a largeapparent change of receiver position.

U.S. Pat. No. 5,752,513 to Acker, et al. depicts a system which is asubset of the system described by Blood ′330, and operation in allrespects is identical with respect to non-dipole transmitter propertiesand metal sensitivity.

U.S. Pat. No. 5,550,091 to Fukuda, et al. depicts a system using aso-called “Helmholtz” arrangement to produce a controlled field withinthe operating volume. One disadvantage of this system is its bulk,requiring the operating volume to be surrounded by the “Helmholtz” coilassembly. A second disadvantage of this system is that, when placed upona metallic object such as a steel table, the magnetic field from thetransmit coils will be distorted inside of the operating volume.

U.S. Pat. No. 5,640,170 to Anderson discloses a method of positioning adipole over a specially constructed spiral over a ground plane. Thedipole transmitter in this system must be located over the center of thespiral ground plane assembly, which makes patient placement moredifficult in a clinical setting, as this placement may interfere withthe surgical field during certain procedures. The benefit of this methodis that it is possible to locate the transmitter closer to the groundplane, and one does not need to use the “method of images” to solve forposition, but the disadvantage of transmitter location over thespiral/ground plane assembly is very similar to the case of a groundplane only.

U.S. Pat. No. 5,198,768 to Keren depicts a surface coil array for use inNMR applications. The system does not determine position, and does notutilize any methods for reducing the effect of nearby metallic objects.

The present invention represents a radical departure from the prior artrelating to such transmitting and receiving position and orientationdevices insofar as it is capable of satisfying the requirement ofinsensitivity to metallic objects under and adjacent to the transmitterassembly without exhibiting the disadvantages of signal degradation.

SUMMARY OF THE INVENTION

The present invention relates to embodiments of a magnetic fieldposition and orientation measurement system with means for substantiallycontaining, confining and re-directing the magnetic field from one ormore transmit elements such that the fields are attenuated in areasoutside of the operating volume in areas where metallic objects arecommonly found.

The present invention relates to devices for measuring the position ofreceiving antennae relative to transmitting antennae using magneticfields. Particularly, although not exclusively, such devices are formeasuring that position in six degrees of freedom, namely, motion ortranslation in three coordinate directions (location) and rotationalmotion above three coordinate axes (orientation), location beingcommonly defined by X, Y, and Z linear coordinates referring to threemutually perpendicular directions and orientation being commonlydescribed by pitch, roll and azimuth angular coordinates above threemutually perpendicular axes usually coincident with the three mutuallyperpendicular directions.

The present invention includes the following interrelated objects,aspects and features:

(1) In the preferred embodiment, a flux containment means is used tore-direct the flux vectors such that they are enhanced inside of thesensor operating volume and decreased under and adjacent to thetransmitter plane, which reduces the sensitivity of the system to metalsunder and near the transmitter. The flux vectors from the transmittersare distorted by the flux containment means in a stable and repeatablemanner, thus it is possible to precisely and repeatably characterize thedistorted field. Once the precise vector distribution from thetransmitter assembly is known, solution of position and orientation froma receiving means is a straightforward task to those familiar with themagnetic position tracking art. One reliable method for accomplishingthis vector characterization is to utilize finite element analysis tocompute the magnetic field vectors from the transmitter. Anotherreliable method is to employ one of several so-called mapping techniqueswhich are known processes to those familiar with the art.

(2) The preferred embodiment of the present invention teaches a methodfor creating a representative magnetic transmitter assembly with reducedsensitivity to metallic objects under and adjacent to the operatingvolume of the system. The preferred embodiment also reduces the vectordilution effects of a conductive ground plane to levels which are nolonger of concern. This reduction in vector dilution yields a systemwhich is substantially less sensitive to distortion caused by metallicobjects within the operating volume while maintaining insensitivity tometallic objects below the transmitter and reduced sensitivity toobjects adjacent to the operating volume. The transmit means may includewire loops, solenoids, or permanent magnets arranged in convenientshapes and locations for determining the position of the receiver withinthe volume.

(3) The present invention achieves the requirement for a system whichmay be placed upon a surface of any extent and composition withoutdegrading the accuracy of the position readings from a sensor locatedwithin the desired operating volume. It achieves this goal for both ACand DC transmitter excitations, which is not at all possible using priorart ground plane based compensation methods. It achieves this goal whilesignificantly increasing the magnetic field intensity within theoperating volume, which is not possible using prior art ground planebased compensation methods. It also avoids the problem of vectordilution which is introduced when a conductive ground plane is placednear the transmitter.

(4) In the preferred embodiment of the present invention, a thin,permeable barrier made of a highly permeable but substantiallynon-conductive material such as ferrite or mumetal is placed on top of aconductive plate. In the preferred embodiment, the thickness of thepermeable layer when made of ferrite is from 0.05 inches to 0.25 incheswhereas use of mumetal can reduce the thickness to below 0.01 inches.The conductive plate, preferably made of an aluminum alloy, may be from{fraction (3/16)} of an inch to ¼ inch in thickness. Where mumetal isemployed in the permeable layer, the thickness of the conductive platemay be reduced because the thickness is not chosen for mechanicalsupport. On top of the ferrite or mumetal barrier, a planar rhombicthree axis transmitter is placed, details of which are presented in U.S.Pat. 5,600,330. In the preferred embodiment, the transmitter consists ofa PC board with the transmitter etched thereon. PC boards havingthicknesses varying from 0.03125-0.125 inches may be employed.

(5) If desired, the permeable barrier may have a flat, planarconfiguration. Alternatively, it may be made to resemble, incross-section, a cake pan having a flat central region with upliftedperipheral edges. Alternatively, the permeable barrier may have agenerally flat configuration with peripheral edges that taper outwardlyfrom the top surface thereof to the bottom surface thereof with thetaper making an angle with the bottom surface in the range of,preferably, 3° to 85°.

(6) If a conductive object in the regions adjacent to or under thetransmitter is subjected to an AC magnetic field, an eddy current willbe induced in the object. This induced eddy current will produce amagnetic field component, which, by the addition of vectors, willcombine with and distort the normal metal-free magnetic field near theobject. The magnitude of this parasitic eddy field is proportional tothe magnitude of the AC field near the conductive object.

(7) It is thus seen that if the field vectors in the operating volumeabove the transmitter assembly remain constant in magnitude anddirection while the field magnitude in the regions adjacent to and underthe transmitter assembly are reduced, then metallic objects in thoseregions will have a proportionally reduced distorting effect on thefield in the operating volume above the transmitter assembly. If thefield magnitude in the operating volume above the transmitter assemblyis increased while the field magnitudes in the regions adjacent to andunder the transmitter assembly remain constant, the distortion reducingeffect is similar. Accordingly, the ratio of the magnetic fieldamplitude in the operating region above the transmitter assembly overthat of the regions adjacent to and under the transmitter assembly maybe used to predict sensitivity to metallic objects. A similardescription applies to ferromagnetic distortion effects when thedistorting objects are located in the regions adjacent to and below thetransmitter assembly.

(8) If the relative magnetic distortion sensitivity values of a singletransmit coil in the configuration such as is shown in FIG. 13, can beestablished as a normal value, then a relative distortion sensitivityfigure of merit Ma for objects adjacent to the operating volume may bedefined where Ma equals (the field of the system depicted in FIG. 2 inthe region above the transmitter assembly) divided by (the field of thesystem depicted in FIG. 5 in the region above the transmitter assembly)divided by (the field of the system illustrated in FIG. 2 in the regionadjacent the transmitter assembly) divided by (the field of the systemin the configuration of FIG. 5 in the region adjacent the transmitterassembly). The system depicted in FIG. 11 will have a sensitivity figureof merit of 1 in that FIG. 11 will, for example, be chosen as thereference system.

(9) Similarly, for comparison of objects below the transmitter assembly,we can define a term Mb which equals (the field of the system of FIG. 2in the region above the transmitter assembly) divided by (the field ofthe system illustrated in FIG. 5 in the region above the transmitterassembly) divided by (the field of the system of FIG. 2 below thetransmitter assembly) divided by (the field of the system of FIG. 5 inthe region below the transmitter assembly). Using the figures of meritMa and Mb, several different configurations can be evaluated todetermine likely relative sensitivities to metallic objects in theregions adjacent to and below the transmitter assembly.

Accordingly, it is a first object of the present invention to provide amagnetic position measurement system with field containment means.

It is a further object of the present invention to provide such a systemwherein a thin, permeable barrier is mounted above a thin, conductiveplate.

It is a still further object of the present invention to provide such asystem wherein the substantially high permeability, substantiallynon-conductive barrier has upturned peripheral edges.

It is a still further object of the present invention to provide such asystem wherein the substantially high permeability, substantiallynon-conductive barrier has peripheral edges that taper downwardly from atop surface thereof to a bottom surface thereof.

It is a still further object of the present invention to provide such asystem wherein a thin, rhombic transmitter is mounted above thepermeable barrier.

It is a still further object of the present invention to provide asystem for quantitatively measuring the position of receiving antennaerelative to transmitting antennae without encountering the disadvantagesthat accrue from sensitivity to metallic objects directly below thetransmitter.

It is a yet further object of the present invention to create a systemthat is insensitive to metallic objects at or below the plane of thetransmitter and extending horizontally as far as possible.

It is a still further object of the present invention to provide such asystem which avoids loss of transmit field intensity within the intendedoperating volume.

It is a still further object of the present invention to provide such asystem which is not significantly degraded in performance by vectordilution effects.

It is a yet further object of the present invention to provide such asystem which may use either DC or AC transmitter excitation techniquesand which is insensitive to magnetic objects placed below thetransmitter configuration.

These and other objects, aspects and features of the present inventionwill be better understood from the following detailed description of thepreferred embodiments when read in conjunction with the appended drawingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a prior art system.

FIG. 2 shows a perspective view of a preferred embodiment of the presentinvention with the rhombic transmitter being shown schematically.

FIG. 3 shows a side view of the preferred embodiment illustrated in FIG.2.

FIG. 4 shows a side perspective view of the preferred embodiment ofFIGS. 2-3, also showing a patient supported above the preferredembodiment on an operating table.

FIG. 5 shows the regions chosen to predict metal sensitivity of a priorart non-dipole transmitter placed over a conducting ground plane.

FIG. 6 shows regions chosen to predict metal sensitivity for theinvention illustrated in FIGS. 2-4.

FIG. 7 shows the regions chosen to predict metal sensitivity for thetransmitter as depicted in FIG. 3 but without the conductive materialplate under the permeable barrier.

FIG. 8 shows a modification of the present invention employing anon-planar, permeable barrier.

FIG. 9 shows a further modification wherein the transmitter extendsbeyond the periphery of the permeable barrier.

FIG. 10 shows the magnetic flux pattern wherein the transmittercomprises a dipole transmitter.

FIG. 11 shows the regions chosen to predict metal sensitivity for amodification of the permeable barrier wherein the periphery has raisededges.

FIG. 12 shows a further variation wherein the transmitter is raisedabove the permeable barrier.

FIG. 13 shows the regions chosen to predict metal sensitivity for aprior art non-dipole transmitter loop in free space.

FIG. 14 depicts the system wherein a dipole magnetic transmitter islocated above a permeable barrier.

FIG. 15 shows a modification of the permeable barrier having peripheraledges that taper outwardly from a top surface thereof to a bottomsurface thereof.

FIG. 16 shows the field equipotential contour cross-section extendingabove and below a reference line as generated at the point 0, 0.

FIG. 17 shows a graph of the same field equipotential contourcross-section shown in FIG. 16 truncated below the reference linethrough employment of a permeable barrier.

FIG. 18 shows a graph of the same field equipotential contourcross-section shown in FIG. 17 but further truncated through addition ofa conductive plate.

SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference, first, to FIG. 1, a system generally designated by thereference numeral 1 is seen to include a prior art three axis dipoletransmitter designated by the reference numeral 2 and seen suspendedover conductive plate 3. Eddy currents induced in the conductive plate 3due to the X and Y coils of the transmitter 2 are nearly identical withrespect to magnitude, direction and distribution on the conductive plate3. This similarity causes the eddy current magnetic field vectors atpoints inside of the operating volume 4 to be quite similar in bothmagnitude and direction. As the transmitter 2 is moved closer to theconductive plate 3, the magnitudes of the eddy current fields relativeto the transmitted fields at any point inside of operating volume 4 willincrease. Since the eddy current fields from conductive plate 3 aresimilar, this causes the total magnetic field vectors from the X and Ycoils to become more similar as well, which reduces the angle formed bythe intersection of these two vectors. Magnetic dipole systems utilizethe angle of intersection of the three distinct vectors from the threeorthogonal transmitter coils to derive orientation. If these anglesbecome distorted due to the presence of eddy current fields, the systemwill output orientation values that are distorted as well.

In order to remove such distortion, in the prior art, such errors areremoved using a field mapping process familiar to those skilled in theart. However, such a field mapping process has a serious drawback whenapplied to a system such as that which is illustrated in FIG. 1. For agiven amount of error in determining the three intersection angles, thesystem will output an error in the orientation output. If this error isdue to a noise source, the orientation output will become noisy. As theangles of intersection of the transmitted vectors from the X, Y, and Zcoils are reduced, receiver orientation determination becomes moresensitive to noise and other errors. In the extreme case where they arenearly identical and the angles of intersection are nearly zero,orientation determination becomes impossible as sensitivity to errorsand noise approaches infinity.

A method of comparing predicted magnetic field distortion levels for agiven metallic environment is useful when evaluating different systems.One such method utilizes magnetic field intensity ratios. Such a ratiois defined as the strength of the magnetic field in an area wheremeasurements are to be taken divided by the strength of the magneticfield at an area outside of said area. The latter volume is typicallychosen as a volume immediately adjacent to the prior volume. To furtherfacilitate this analysis, a single point is chosen to represent theentire magnetic field within the respective volumes. The theoreticalbasis for this method follows.

If a conductive object in the regions adjacent to or under thetransmitter is subjected to an AC magnetic field, an eddy current willbe induced in the object. This induced eddy current will produce amagnetic field component, which, by the addition of vectors, willcombine with and distort the normal metal-free magnetic field near theobject. The magnitude of this parasitic eddy field is proportional tothe magnitude of the AC field near the conductive object.

It is thus seen that if the field vectors in the operating volume abovethe transmitter assembly remain constant in magnitude and directionwhile the field magnitude in the regions adjacent to and under thetransmitter assembly are reduced, then metallic objects in those regionswill have a proportionally reduced distorting effect on the field in theoperating volume above the transmitter assembly. If the field magnitudein the operating volume above the transmitter assembly is increasedwhile the field magnitudes in the regions adjacent to and under thetransmitter assembly remain constant, the distortion reducing effect issimilar. Accordingly, the ratio of the magnetic field amplitude in theoperating region above the transmitter assembly over that of the regionsadjacent to and under the transmitter assembly may be used to predictsensitivity to metallic objects. A similar description applies toferromagnetic distortion effects when the distorting objects are locatedin the regions adjacent to and below the transmitter assembly.

If the relative magnetic distortion sensitivity values of a singletransmit coil in the configuration such as is shown in FIG. 13, can beestablished as a normal value, then a relative distortion sensitivityfigure of merit designated Ma may be arrived at where Ma equals (thefield of the system depicted in FIG. 2 in the region above thetransmitter assembly) divided by (the field of the system depicted inFIG. 5 in the region above the transmitter assembly) divided by (thefield of the system illustrated in FIG. 2 in the region adjacent thetransmitter assembly) divided by (the field of the system in theconfiguration of FIG. 5 in the region adjacent the transmitterassembly). The system depicted in FIG. 11 will have a sensitivity figureof merit of 1 in that FIG. 11 will, for example, be chosen as thereference system.

Similarly, for comparison of distortion sensitivity to metallic objectsbelow the transmitter assembly, we can define a term Mb which equals(the field of the system of FIG. 2 in the region above the transmitterassembly) divided by (the field of the system illustrated in FIG. 5 inthe region above the transmitter assembly) divided by (the field of thesystem of FIG. 2 below the transmitter assembly) divided by (the fieldof the system of FIG. 5 in the region below the transmitter assembly).Using the figures of merit Ma and Mb, several different configurationscan be evaluated to determine likely relative sensitivities to metallicobjects in the regions adjacent to and below the transmitter assembly.

With reference to FIG. 13, a further prior art system is shown wherein asingle turn transmit coil 6 having a diameter of 7.5 inches operates ata frequency of 20 KHz and with a current of 1 ampere rms. The magneticfield vectors may be calculated using a number of methods, one of whichis known as the so-called finite element method. The primary tool forthis calculation is a software program employing Maxwell's equations asboundary conditions and field properties. Using a computerized draftingprogram, the model to be analyzed is created. The model consists of boththe geometry and material properties of the system, as well as anyexcitation properties. This model is then operated on by a numericalfinite element solver, which simulates the behavior of electromagneticfields in and around the model. The result is an accurate, quantifiedvalue for the magnetic filed at all points of the model. By judiciouscreation of the models, easily accomplished by those skilled in the art,it is possible to analyze various combinations of materials andgeometry. It is further possible to choose an exact spatial location onthe model and obtain an accurate numerical figure for the magnetic fieldvector magnitude and direction at that location. By choosing these samelocations and varying model parameters, it is possible to view theeffects of material properties and geometries on the magnetic field at aparticular spatial location. A point within each of operating volume 7,adjacent space 8 and region 9 below the transmit coil is chosen torepresent field behavior within the respective volume. A geometricorigin of 0, 0, 0 is chosen at the center of transmit loop 5. Operatingvolume 7 is represented by a point of (0, 0, 6). Adjacent space 8 isrepresented by a point of (0, 12, 0). Region 9 is represented by (0, 0,6). Utilizing this method, the magnetic field vector magnitudes foroperating volume 7 are (9.12e-17 Tesla), the adjacent space 8 is(7.2e-17 T) and the region 9 below the transmit coil 6 is (6.4e-15 T).This configuration is chosen as a reference configuration and,accordingly, Ma and Mb are equal to 1.

With reference, now, to FIG. 5, a further prior art system is shown inwhich a flat transmitter 5 is suspended 0.3 inches over the conductiveplate 3. In this prior art teaching, the conductive plate 3 isapproximately 0.25 inches thick and made from aluminum. Thus, theoverall height from the top surface of the conductive plate 3 to the topof the transmitter 5 is 0.55 inches.

In comparing the configurations of FIGS. 5 and 13, looking at theconfiguration of FIG. 5, compared with the field levels of theconfiguration of FIG. 13, the operating volume field 7 is reduced to27%, adjacent space 8 has been reduced to 40%, and the region 9 belowthe conductive plate 3 has been reduced to 0.14%. From this data, it canbe concluded that Ma equals 0.68 and Mb equals 193. A value of Ma equals0.68 indicates that this system is likely to experience greaterdistortion due to metallic objects within the region 8. The Mb valueindicates that the system will be quite insensitive to metallic objectslocated below the conductive plate 3 within the region 9. A seriousdisadvantage in the configuration of FIG. 5 is that the field operatingvolume 7 has been reduced to 27% of its original value. This means thatfor a given noise level in the tracking system and its environment,position output will necessarily be degraded. Increasing transmittercurrent by a factor of 3.6 can compensate for this loss, but this willresult in a more robust and costly drive system. Also, if the transmitloop 5 is not a superconductor, it will dissipate power equal toI{circumflex over ( )}2R or 12.9 times more power for a given transmitloop configuration. This may require a larger conductor size and/orprovisions to remove heat from the conductor of transmit loop 5, both ofwhich provide distinct disadvantages. The system described in FIG. 5also suffers from substantial vector dilution effects due to similareddy current fields from conductive plate 3 when any of the threetransmit loops are energized. Thus, the conductive plate 3 is moresensitive to metallic objects inside of operating volume 7.

With reference, now, to FIGS. 2, 3 and 4, the preferred embodiment ofthe present invention is generally designated by the reference numeral10 and is seen to include a planar rhombic transmitter 11 located over apermeable barrier 13 mounted on top of a conductive plate 15.

In the preferred embodiment, the transmitter 11 consists of a PC boardhaving the three axis transmitter etched onto the surface thereof. Inthe preferred embodiment, the PC board has a thickness of 0.0625 inches,although PC boards having thicknesses from 0.03125 to 0.125 inches inthickness may be suitably employed.

For proper operation, the permeable barrier should not be a major sourceof eddy current field. For a given material having a bulk resistivity p,as the frequency is decreased a point is reached where the eddy currentin the material is reduced to the point where the distortion to theincident magnetic field becomes small. In the extreme case of a DCtransmitter, it can be seen that the conductivity of the permeablebarrier is not of concern. It is apparent that there is a relationshipbetween material conductivity and frequency which is of use whendetermining the frequency of operation of the transmitter and also whenselecting the required bulk resistivity of the permeable barrier. Thiscan be described by the bulk resistivity p of the material in ohm metersdivided by the frequency of operation f of the transmitter, defined asRfc=p/f. For steels, when Rfc is greater than about 2e-10, the parasiticeddy current field from the permeable barrier is low enough such that itis beneficial to use the steel barrier versus an aluminum or copperground plane. For cold rolled steel, this occurs at a transmit frequencyof about 500 Hz. When Rfc is greater than 2e-9, it is generallybeneficial to use steel or stainless steel over ferrite unless it isabsolutely required to fully optimize the transmit fieldcharacteristics. For cold rolled steel, this occurs at a transmitfrequency of about 50 Hz. When Rfc is greater than 1e-8, the barrier isacting as an essentially pure permeable barrier when the material issteel or stainless steel. For cold rolled steel, the transmitterfrequency would be 10 Hz. In this case, replacing the inexpensive andstrong steel with expensive and fragile ferrite would produce noperformance improvement.

The permeable barrier 13 may be from 0.05 to 0.25 inches in thickness,although a thickness range from 0.15 to 0.25 inches is preferred. Thepermeable barrier 13 may be made of a highly permeable but substantiallynon-conductive material. One such material is ferrite. This material hasa relative permeability range of 50 to 25,000 compared with thepermeability of air. This material has a typical resistivity on theorder of 0.1 Ohm/meter to 10{circumflex over ( )}8 Ohm/meter, dependingon the commercial formulation used. In a specific implementation of thepreferred embodiment, a suitable material may be ferrite type MN67 whichhas a resistivity of 25 degrees C. of 10{circumflex over ( )}4Ohms/meter and a relative permeability of 2500 at 25 degrees C. Thematerial is 0.2 inches thick and 18 inches in diameter and is concentricabout the transmitter 11 formed by three rhombic transmit loops 14, 16and 18. As seen in FIG. 2, a transmit driver 21 is connected to thetransmitter 11 via an electrical conductor 23. The driver 21sequentially energizes each of the loops of the transmitter 11 with aone ampere r.m.s. at a frequency of 20 Khz. A further material to beemployed for the permeable barrier is mumetal. This material is a NickelIron alloy containing small amounts of other metals. It is speciallyformulated and annealed to provide a relative permeability Ur of 75,000to 300,000, although technically conductive the enhanced permeabilityover that of ferrite compensates for this aspect and the mumetal hasproven to be a highly effective permeable barrier. A representativecommercial product is named AD-MU-80 mumetal, and is made by Ad-vanceMagnetics, Inc. In an experiment, a 0.010 inch thick sheet of thismaterial was employed as the ferrite barrier 13 and the effects on themagnetic field of the transmitter were analyzed. It was found that attransmitter frequencies of DC to 3 KHz, AD-MU-80 mumetal providedperformance substantially equal to that of the 0.2 inch thick MN-67ferrite material. At frequencies from 3 KHz to 19 KHz, AD-MU-80 mumetalprovided the same percentage reduction in adjacent field strength andbelow field strength as MN-67 ferrite, but provided less of a fieldstrength increase in the operating region. At frequencies above 19 KHz,AD-MU-80 mumetal provided the same field reductions in the below andadjacent regions as MN-67 ferrite, but also reduced the field strengthin the operating region. At all frequencies tested, which include DC and5 MHz, AD-MU-80 mumetal produced significantly lower vector dilutioneffects and significantly higher transmitter field strength in theoperating region than a conductive ground plane.

Mumetal has mechanical properties that are very useful as compared toferrite. Since it typically has about 30 times the permeability offerrite at frequencies below a few Khz, it can be made much thinner thanferrite while performing equally well as a permeable barrier. Unlikeferrite, mumetal is not a brittle ceramic material but is instead aductile metal. This allows the rigid support backing required forferrite to become comparatively thin or non-existent, as mumetal willnot fracture when stressed as will ferrite. Since the permeable barrier13 may be made thinner, a weight savings may be realized over ferrite,with obvious benefits. Also, mumetal is much less expensive thanferrite, and may easily be shaped, formed, machined, and welded intoconvenient shapes to form the permeable barrier 13. As a result of theseadditional benefits, mumetal may be useful in replacing ferrite as thepermeable barrier 13 even in cases where it provides lower performancegains, as economic and mechanical considerations may offset theperformance difference.

The conductive plate 15 is located directly below and substantially incontact with the permeable barrier 13. In the preferred embodiment, theconductive plate 15 is made of aluminum alloy 6061 T-6 and has athickness of approximately 0.1875 to 0.25 inches.

Thus, the combination of the transmitter 11, permeable barrier 13, andconductive plate 15 has a combined thickness of approximately 0.3 to0.625 inches, a quite compact assembly. The combination of thetransmitter 11, permeable barrier 13, and conductive plate 15 may begenerally referred to as transmitter assembly 25.

FIG. 4 shows the transmitter assembly 25 mounted on a surgical table 27with a patient 30 lying on the transmitter assembly 25. A receiver 31has been inserted into the body of the patient 30 and receives signalsfrom the transmitter assembly 25 conveying them to a computer (notshown) via the electrical conductor 33 so that the position andorientation of the receiver 31 may be accurately determined.

FIG. 16 shows a graph of a magnetic field equipotential contourcross-section 37 emanating from the point 0, 0 with the field extendingabove and below the x-axis line 39. By contrast, with reference to FIG.17, when a permeable barrier 25 is placed on the line 39, the magneticfield equipotential contour cross-section 37 is changed in shape so thatvirtually none of the field 37 extends below the line 39. FIG. 18 showsfurther attenuation of the field equipotential contour cross-section 37below line 39 through addition of conductive plate 41 below permeablebarrier 25. This effect is what occurs through operation of thepreferred embodiment illustrated in FIGS. 2, 3 and 4.

With reference to FIG. 6, an embodiment of the transmitter assembly 25is shown wherein the transmitter 11 is located directly on top of aferrite layer 13 made of MN67 ferrite material 0.2 inches thick, whichferrite layer 13 lies directly on top of a 0.25 inch thick aluminumconductive plate 15. Compared to the field levels exhibited withreference to FIG. 13 as described above, the operating volume 7 is 159%of that of FIG. 13, the adjacent space 8 is 60% of that of FIG. 13, andthe region 9 below the transmitter assembly 25 is 0.11% of that of FIG.13. From these results, it is clear that Ma equals 2.65 and Mb equals1445. Thus, it should be understood that the configuration of FIG. 6performs better than the systems of FIGS. 13 and 5 with respect topredicted sensitivity to metallic objects in the regions 8 and 9. Thesignal level is also increased within the operating volume 7 by 151% ascompared to FIG. 13.

FIG. 7 depicts the transmitter 11 located directly on top of the ferriteplate 13 with the ferrite plate being 0.2 inches thick and beingcomposed of type MN67 ferrite. The overall thickness of the transmitterassembly in FIG. 7 is 0.2 inches. Compared to the field levels of FIG.13, the operating volume field 7 is 191%, the adjacent space 8 is 81%and the region within and under the table 9 is 4.3%. It follows fromthis data that Ma equals 2.35 and Mb equals 44.4. From this data, it ispredicted that this system will be significantly less sensitive tometallic objects in the region 8 and much less sensitive to metallicobjects in the region 9 as compared to FIG. 13. Also, the field inoperating volume 7 has been increased by 191% over the original whichwill result in improved signal-to-noise performance. Vector dilution isnegligible. Although the signal level in operating volume is only 83% ofthat of FIG. 7, the FIG. 6 system is, in practice, much better suited toapplications where the region 9 consists of the region within and belowthe operating table since the system will not experience significantmagnetic field distortion in the region 7 when the composition of theregion 9 is varied. Vector dilution is negligible so that sensitivity tometallic objects within the operating volume 7 is not diminished overFIG. 13.

TABLE 1 Comparison of Ma, Mb and Operating Volume Magnetic FieldStrength for 4 Representative Planar Non-Dipole Magnetic Transmitters

TABLE 1 Comparison of Ma, Mb and Operating Volume Magnetic FieldStrength for 4 Representative Planar Non-Dipole Magnetic TransmittersOperating Volume Magnetic Field Referenced to that SYSTEM OF Ma Mb OfFIG. 13  FIG. 13 1 1 1 FIG. 5 .68 193 .27 FIG. 6 2.65 1445 1.59 FIG. 72.35 44.4 1.91

A further benefit of the conductive plate 15 is that it provides aphysical mechanical support to the ferrite layer 13 which is typicallyquite fragile. Of course, additionally, undesirable signal loss effectsof the eddy current effects from conductive plate 15 are substantiallyeliminated. Ideally, the conductive plate 15 is chosen to be severalskin-depths thick at the frequency of operation to provide a maximumdegree of field attenuation at the bottom of the transmitterconfiguration 25. In the case of very low frequency excitation,including DC excitation, where skin depths become very large, thepurpose for the conductive plate 15 becomes purely for mechanicalsupport of the transmitter.

Applicant has found that a non-dipole system may be enhanced inperformance through operation of the present invention. The enhancednon-dipole shows increased magnetic field strength within the operatingvolume with an accompanying decrease in output noise. It is, in apractical sense, totally insensitive to metallic objects located beneaththe transmitter, for example, in the region designated by the referencenumeral 9. Such a system shows reduced sensitivity to metallic objectsadjacent to the operating volume and also has reduced vector dilutioneffects as compared to a ground plane-based shielding method and is thusinherently less sensitive to metallic objects within the operatingvolume and also less sensitive to noise.

FIG. 8 shows an alternative permeable barrier 50 that is non-planar inconfiguration having a shallow V-shaped cross-section, consisting of twoportions 51 and 53 meeting at a line of intersection 55. The portions 51and 53 make an angle of 15 degrees with respect to horizontal and angledownwardly from a central upper terminus. The transmitter 57 issuspended thereabove as shown in FIG. 8.

FIG. 9 shows a further embodiment of the present invention designated bythe reference numeral 60 wherein the permeable barrier 61 has atransmitter 63 suspended thereover with the peripheral edges 65, 67 ofthe transmitter 63 overlying the peripheral edges 62 and 64 of thepermeable barrier 61.

FIG. 10 shows a transmitter 70 suspended above a permeable barrier 71and depicts the magnetic flux pattern for this configuration.

FIG. 11 shows a system 80 having a permeable barrier 81 with upturnedperipheral edges 83 so that the cross-section thereof resembles a cakepan. The transmitter 85 is suspended within the volume created by theperipheral edges 83. Applicant has found that when using a permeablebarrier such as that which is depicted by the reference numeral 81, themagnetic field concentrates about the upper edges thereof, providingcertain advantages when the configuration is placed on a ferromagneticsheet, such as plate steel. The advantage of a thin transmitter issomewhat compromised in this case and the field shape around the raisedperipheral edges 83 is also changed as is the intensity distribution.

In a further modification, reference is made to FIG. 15 which shows aferrite permeable barrier 90 having a main body 91 and peripheral edges93 that are tapered outwardly from a top surface 94 to a bottom surface95 of the barrier 91. The peripheral edges 93 make an angle that ispreferably in the range of 30 to 85 degrees. As the angle reduces, theperformance results improve, however, one arrives at the point ofdiminishing returns as the angle is reduced for two reasons. First, itbecomes more and more difficult to manufacture the barrier 91 with theshallower angled peripheral edges 93. Furthermore, once one reduces theangle of the peripheral edges from the typical 90 degrees to 45 degrees,one has achieved about 99% of the enhancement that is possible toachieve. Using such an angled peripheral edge reduces the magnitude ofthe fringe effects at the edge. The bevel of the angled peripheral edge93 moves distortions closer to the edge of the permeable barrier 91 andfurther downward into, for example, the operating table adjacent theinventive system.

FIG. 12 shows a further modification of the assembly illustrated inFIGS. 2-4 in which the transmitter coils 11 are suspended above thepermeable barrier 13.

FIG. 14 depicts the fact that use of the permeable barrier 13 reducesvector dilution effects of the ground plane by providing a lowreluctance flux path for the magnetic field emitted by the transmitter2. This effectively attenuates the magnetic field which is incident uponthe conductive plate 15 to an insignificant level with the result thatvector dilution effects are greatly reduced while maintenance ofinsensitivity to metallic objects below the transmitter occurs. Whilethe permeable barrier 13 distorts the transmitted fields from the X, Y,and Z coils, the distortion is not severe and is easily removed usingfield mapping techniques.

Applicant has found that use of the ferrite barrier, with its extremelylow reluctance, causes the magnetic field to travel primarily throughthe low reluctance path provided by the ferrite material effectivelyshielding objects below the ferrite material. The advantages of the useof mumetal for the permeable barrier have been explained in detailhereinabove.

Use of aluminum for the conductive plate is advantageous becausealuminum attenuates the magnetic field in addition to providing asupport for the brittle ferrite permeable barrier. Applicant has foundthat the ferrite or mumetal permeable barrier provides 95% of thebenefit of the present invention with modifications and variationsdisclosed herein providing the other 5% of the benefit, to wit, suchthings as the shape of the periphery of the permeable barrier and theuse of the aluminum conductive plate.

One typical application intended for the present invention is on top ofan operating table. Operating tables have a lot of steel in them and areheavily cantilevered. The present invention amplifies the field in theoperating region above the table and reduces the field next to thetransmitter and below the top surface of the operating table.

If desired, the transmitter 11, permeable barrier 13, and aluminumconductive plate 15 may be laminated together with a material such assilicon or epoxy adhesive. As mentioned above, the finished laminatedassembly may have a thickness no greater than ⅝ of an inch making it aconvenient enhancement to any operating room.

As such, an invention has been disclosed in terms of preferredembodiments thereof which fulfill each and every one of the objects ofthe invention as set forth hereinabove and provide a new and usefulmagnetic position measurement system with field containment means ofgreat novelty and utility.

Of course, various changes, modifications and alterations in theteachings of the present invention may be contemplated by those skilledin the art without departing from the intended spirit and scope thereof.

As such, it is intended that the present invention only be limited bythe terms of the appended claims.

What is claimed is:
 1. In a magnetic position measurement system, theimprovement comprising means for containing a magnetic field used toconduct measurements of position of an object in three dimensions, saidcontaining means comprising a magnetic field permeable attenuatorlocated adjacent a region where position of said object in threedimensions is being measured by a magnetic field, said attenuatorattenuating said magnetic field on a side of said attenuator remote fromsaid region, said system including a three axis transmitter engagingsaid attenuator on a side thereof opposite said remote side.
 2. Thesystem of claim 1, wherein said attenuator is flat.
 3. The system ofclaim 2, wherein said attenuator has a uniform thickness of 0.01 to 0.25inches.
 4. The system of claim 3, wherein said attenuator is made of amaterial chosen from the group consisting of ferrite and mumetal.
 5. Thesystem of claim 4, wherein said attenuator has an upraised peripheraledge.
 6. The system of claim 2, wherein said attenuator has an upraisedperipheral edge.
 7. The system of claim 2, wherein said attenuator has aperipheral edge tapered outwardly from a top surface of said attenuatorto a bottom surface thereof.
 8. The system of claim 4, wherein saidattenuator has a peripheral edge tapered outwardly from a top surface ofsaid attenuator to a bottom surface thereof.
 9. The system of claim 1,wherein said attenuator has a V-shaped cross-section.
 10. The system ofclaim 9, wherein said attenuator has a uniform thickness of 0.01 to 0.25inches.
 11. The system of claim 10, wherein said attenuator is made offerrite.
 12. The system of claim 10, wherein said attenuator is made ofmumetal.
 13. The system of claim 1, further including a conductive plateattached under said attenuator.
 14. The system of claim 13, wherein saidplate has a thickness of 0.1875 to 0.25 inches.
 15. The system of claim14, wherein said plate is made of non-ferrous metal.
 16. The system ofclaim 14, wherein said plate is made of a conductive metal.
 17. Thesystem of claim 16, wherein said conductive metal is non-ferrous. 18.The system of claim 1, further including a three axis transmittermounted on top of said attenuator.
 19. The system of claim 18, whereinsaid transmitter comprises a PC board with a transmitting means etchedthereon.
 20. The system of claim 19, wherein said PC board is 0.03125 to0.125 inches thick.
 21. The system of claim 3, wherein said magneticfield is created by a pulsed DC power source.
 22. A magnetic positionmeasurement system, comprising: a) a thin magnetic field permeableattenuator; b) a thin conductive plate below said attenuator; c) a thintransmitter above said attenuator, said transmitter capable of measuringin three dimensions; d) said transmitter, attenuator and plate beinglaminated together.
 23. The system of claim 22, wherein saidtransmitter, attenuator and plate have a combined thickness of 0.3 to0.625 inches.
 24. The system of claim 22, wherein said transmittercomprises a PC board with a transmitting means etched thereon.
 25. Thesystem of claim 22, wherein said attenuator is made of a material chosenfrom the group consisting of ferrite and mumetal.
 26. The system ofclaim 22, wherein said plate is made of aluminum.
 27. A method ofmeasuring position of an object in a prescribed three dimensional spaceincluding the steps of: a) defining a three dimensional space; b)locating a magnetic field permeable attenuator adjacent said space; c)placing a flat three axis transmitter on a side of said attenuatorfacing said space; d) operating said transmitter; and e) measuringposition of said object.
 28. The method of claim 27, wherein saidlocating step includes the step of providing a attenuator with a uniformthickness of 0.01 to 0.25 inches.
 29. The method of claim 28, whereinsaid providing step includes the step of making said attenuator of oneof ferrite or numetal.
 30. The method of claim 27, further including thestep of installing a conductive plate under said attenuator.
 31. Themethod of claim 30, wherein said installing step includes installing aconductive plate made of aluminum.
 32. The method of claim 30, whereinsaid installing step includes installing a conductive plate having athickness of 0.1875 to 0.25 inches.
 33. The method of claim 30, furtherincluding the step of laminating together said transmitter, attenuatorand plate.